This invention relates to a photosensitive drum for electrographic apparatus composed of a drum-shaped photosensitive body and producing thereon an electrostatic latent image corresponding to a picture image of a manuscript to be reproduced. In an electrographic apparatus for exposing and scanning a picture image of a manuscript to be reproduced for the purpose of producing, on a photosensitive drum composed of a drum-shaped photosensitive body, an electrostatic latent image corresponding to the picture image of the manuscript and for transferring the electrostatic latent image to a record sheet, a carriage on which is placed the manuscript or an optical system for exposing the manuscript is reciprocated so as to expose and scan the manuscript and produce on the photosensitive drum the electrostatic latent image corresponding to the manuscript. The photosensitive drum is mainly composed of a drum-shaped photosensitive body.
In the course of producing, on the photosensitive drum, the electrostatic latent image corresponding to the picture image of the manuscript, it is important to effect the exposure and scanning movement in synchronism with the rotation of the photosensitive drum, that is, not only to make the start and stop positions of both the exposure and scanning movement and the rotation of the photosensitive drum always constant, but also to make the scanning speed equal to the rotary speed of the photosensitive drum.
In general, a gear mechanism for driving both the manuscript carriage or the optical system for exposing the manuscript and the photosensitive drum includes a pulley, wire, gear train, etc. But, the inertia of a gear mechanism for driving the manuscript carriage or the optical system for exposing the manuscript is different from the inertia of a gear mechanism for driving the photosensitive drum. As a result, there is a risk of both the driving mechanisms being subjected to play, motionless tension by backlash, etc. of the gears. In addition, the start and stop positions of both the manuscript carriage or the optical system for exposing the manuscript and the photosensitive drum are not always the same and come out of synchronism, thereby shifting the picture image reproduced.